The present invention relates to valves for the regulation of hydraulic pressure. More particularly, the invention is directed to proportional pressure control solenoid valves.
A conventional method for designing accurate and repeatable pressure regulators which may be subjected to substantial forces generated at operating pressures reaching 2000 psi involves the use of expensive techniques to reduce drag between the solenoid's armature and other magnetic components. This results in a relatively expensive assembly. The typical generation of large forces along the axis of motion of an armature in such conventional devices is usually accompanied by the existence of a large radial force that pulls the armature in a direction perpendicular to the direction of motion. This results in the armature dragging substantially due to the friction existing between the armature and the support bearing. The radial force is directly proportional to the magnitude of the lateral force on the armature and the coefficient of friction between the two surfaces.
This undesirable drag results in a difference in the net axial force produced by the armature on the hydraulic components of a conventional electro-hydraulic pressure regulator in the increasing and decreasing force directions of its operation. This problem has typically been solved by the use of roller or other specialized bearing devices that reduce the effective coefficient of friction between the two surfaces. This conventional solution however, results in a relatively expensive device and complicates the assembly. It also renders these devices sensitive to both contamination and degradation of their repeatability.
In pressure regulators incorporating sliding member valves a conventional method of reducing the inherent leakage through the clearance between the sliding spool and the housing is through the use of very close tolerance components. This tends to render the valves expensive, temperature sensitive and subject to an inherent phenomenon called limit cycling. Limit cycling is associated with devices that have a large change in flow gain as they overcome a positive overlap between the spool and the housing. The limit cycling related initial surge of flow that occurs when such a conventional valve opens is undesirable in many applications.
The typical cost and complication involved in manufacturing a conventional pressure regulator capable of operating with high actuation force and of relatively small size has limited the use of such devices. This in turn has hampered the development of pressure regulator based systems in high pressure applications such as brakes and power steering in the automotive industry.